This invention relates to a method of detecting an abnormal condition in a heat roller and more particularly to a method of detecting an abnormal temperature condition in a rotatable heat roller, for example, of an electrophotographic copier after a specified length of time has passed since the heating of the heat roller was started.
One of the methods of detecting an abnormal temperature condition in the heat roller of an apparatus such as an electrophotographic copier while the apparatus is warming up is to calculate on the basis of a rate of temperature rise the time required for the heater to reach a preset temperature level after its heating is started and then to determine whether this calculated time is within a preset reference time length or not. Another method is to compare the heater temperature measured after a preset period of time with a preset reference temperature. By the first method, the heater is considered to be in an abnormal condition and its power supply is shut off if the calculated time exceeds the preset reference time length. By the second method, an abnormal condition is considered to exist if the measured temperature is higher than the preset reference temperature.
While an apparatus such as an electrophotographic copier is being warmed up, however, not only is its heater preheated but also the driving system for the entire apparatus is operated, for example, to control the toner concentration for the developer unit. When the driving system begins to operate, the heat roller begins to rotate and the rate of temperature increase of the heater is lower when the roller is rotated than when it is stationary. Thus, a correct time required for the heater to reach a preset reference temperature cannot be calculated accurately by the first of the above methods unless the rotation of the heater is taken into consideration. In other words, this method is not reliable unless the time when the driving system stops is also taken into account. This, however, complicates the calculation excessively and the method becomes impractical.
The second of the methods described above is more advantageous because the reference temperature may be set lower than the surface temperature of the heat roller required for its operation and the heater abnormality can be detected more quickly. This method, however, suffers the same problem described above because the driving motor of the system may or may not be operating and hence the heat roller may or may not be rotating during the warmup period of the apparatus and the reference time period must be selected accordingly. In other words, the reference time period must be selected as the time required for the heat roller to reach the specified temperature level when its temperature is rising at a lower rate and this means that it takes longer to conclude whether an abnormality exists or not.